1. Field of the Invention
The present invention relates to a display device having matrix wiring arranged on an insulated electroconductive substrate by an insulator.
2. Related Background Art
The demand for a display device which has lower power consumption and a thinner size has increased with the development of information processing devices, and research and development works for such a display device as to correspond to the demand have been actively conducted.
A display device is often used outdoors, along with increase of such mobile information processing devices as particularly so-called a wearable personal computer and an electronic note, and the reduction of a power consumption and an installation space is expected.
However, many liquid crystal displays have no so-called memory function in general, so that they have to continuously apply voltage on a liquid crystal display panel during the display. On the other hand, a liquid crystal display panel with a memory function has difficulty in sufficiently securing operational reliability, when assuming that it will be used in various environments as are wearable personal computers, so that it has not been practically used yet.
One of a thin-and-light display system with a memory function is an electrophoretic display device disclosed by Harold D. Lees (cf. U.S. Pat. No. 3,612,758).
Electrophoretic display devices of this kind comprise a pair of substrates arranged in a state of being separated by a predetermined gap, an insulating liquid filled in between a pair of the substrates, a lot of coloring chargeable electrophoretic particles dispersed in the insulating liquid, and a display electrode placed on each picture element in parallel to each substrate.
In the display device, the coloring chargeable electrophoretic particles are electrically charged into a positive polarity or a negative polarity, so that they are adsorbed to either of the display electrodes according to the polarity of voltage applied to the display electrode. For instance, between the condition of making the coloring particles adsorbed to an upper electrode to show the coloring chargeable electrophoretic particles and the condition of making the coloring chargeable electrophoretic particles adsorbed to a bottom electrode to show the color of an insulating liquid, through controlling applied voltage, the display device can display various images. Such a type of a display device is called “vertically moving type.”
In addition to the above one, such a type of an electrophoretic display device is disclosed as shown in FIGS. 4A and 4B (cf. Japanese Patent Application Laid-Open No. H9-211499).
The electrophoretic display device has a different configuration from that of the above described type which has an insulating liquid arranged so as to be sandwiched by a pair of substrates, and adopts such a configuration shown in FIGS. 4A and 4B as to have, for instance, a first electrode 231 arranged under a shielding layer (a partition wall) 235 placed between picture elements, and a second electrode 232 which is arranged on the whole picture element display for reflecting incident light and is covered with an insulating film 236.
Accordingly, an insulating liquid 237 has only to be transparent. The electrophoretic display device displays a black color by covering the second electrode 232 with electrophoretic particles 238 as shown in FIG. 4A, and displays a white color by exposing the second electrode 232 through collecting the electrophoretic particles 238 to the first electrode 231 placed between the picture elements as shown in FIG. 4B. Thus, the electrophoretic display device can display images by controlling the polarity of applied voltage for every picture element.
In recent years, a display with a small thickness, hardly being damaged and having flexibility like paper, has been suggested as a display device using the above described electrophoretic method, which could not been conventionally realized. In order to follow the trend, it has been considered to form a TFT (Thin Film Transistor) back plane for driving a display element not on a glass substrate made of conventional glass, but on a comparatively thin metallic sheet or a plastic substrate made of a plastic material.
For instance, a technology is disclosed for forming a TFT back plane on a metallic substrate made of a stainless steel (SUS: Steel Use Stainless) material (cf. Japanese Patent Application Laid-Open No. H9-179106).
As described above, the metallic substrate made of a stainless steel material (SUS) is greatly different from glass materials which have been conventionally used in general and plastic materials having comparatively high flexibility in the respect of having electroconductivity.
FIG. 5 shows one example of a sectional drawing of a pixel electrode in a TFT matrix array formed on a metallic substrate.
As shown in FIG. 5, a conventional display device comprises a substrate-insulating layer 311, a gate electrode 312, a gate insulating layer 314, an amorphous silicon layer 315, an ohmic contact layer 319, a source electrode 320, a drain electrode 317 and a channel protective film 318 sequentially layered on a metallic substrate 310; and an electrode 313 for providing electrical retention capacitance (Cs) in between itself and each of a metallic substrate 310 and a drain electrode 317.
In a conventional display device with the use of a substrate made of a non-conductive material such as glass, reference character a of parasitic capacitance Cg-Sub and reference character b1 or b2 of parasitic capacitance Cd-Sub as shown in FIG. 5 do not exist.
In a display device with the use of an electroconductive substrate as shown in FIG. 5, Cg-Sub of parasitic capacitance a and Cd-Sub of parasitic capacitance b1 or b2 are produced, of which the state is equivalent to such a state that the display device has an electrically connected certain coil component. Then, a metallic substrate in a floating state of having an inconstant substrate potential inconveniently acts as an antenna, picks up noises, adversely affects a pixel circuit formed on a metallic substrate or the driving system of the pixel circuit, and causes malfunction.
In addition, a metallic substrate has a comparatively high linear expansion coefficient in comparison with an AN glass requiring no annealing treatment, which is generally used in a display unit application, and has a large fluctuation of the dimension due to a variation of a heat history in a manufacturing process. Accordingly, in the manufacturing process of a TFT matrix array in particular, the metallic substrate causes difficulty in an alignment step for adjusting the dimension of itself, and makes it difficult to cope with a high-resolution display device, which are problems.
For this reason, objects of the present invention are to provide a panel for a display device, which has an improved reliability of the drive for a pixel circuit and can cope with a high-resolution image, and to provide the display device therefor.